1. Field of the Invention
The present invention relates to a control device for a vehicle including at least an internal combustion engine, a continuously variable transmission, and a stepped variable transmission.
2. Description of Related Art
A hybrid vehicle which includes a power distribution mechanism having an internal combustion engine (hereinafter, referred to as an engine), a first generator motor, a second generator motor, and a planetary gear mechanism is known. A planetary gear of the planetary gear mechanism is rotated directly by the engine through a planetary carrier. A sun gear of the planetary gear mechanism is rotated directly by the first generator motor. A ring gear of the planetary gear mechanism is rotated directly by the second generator motor through a ring carrier and rotates a drive shaft of the vehicle. A rotation shaft of the ring carrier (that is, a member rotating the drive shaft of the vehicle) can be regarded as an output shaft of the power distribution mechanism. A rotation shaft of the planetary carrier (that is, a member rotated directly by the engine) can be regarded as an input shaft of the power distribution mechanism. In this case, the power distribution mechanism can continuously change the ratio of the rotation speed of the input shaft to the rotation speed of the output shaft. Accordingly, the power distribution mechanism can be regarded as a continuously variable transmission.
In a hybrid vehicle in which such a power distribution mechanism is mounted, the engine is operated at an optimum operation point such that fuel efficiency of the engine is optimized. In this case, even if a vehicle speed increases, an engine rotation speed may not be increased, and a driver may feel a sense of discomfort. Even in a vehicle which includes only an internal combustion engine as a vehicle drive source and in which a belt driving type continuously variable transmission (CVT) is mounted, a similar sense of discomfort may be given to the driver. In contrast, Japanese Patent Application Publication No. 2006-51842 (JP 2006-51842 A) discloses a technique which controls a continuously variable transmission and an engine such that an engine rotation speed is increased at a predetermined gradient (an increase rate of the engine rotation speed) with respect to an increase in a vehicle speed when acceleration is requested.
Japanese Patent Application Publication No. 2008-101742 (JP 2008-101742 A) discloses a technique which performs a pseudo gear shift to quickly decrease an engine rotation speed during acceleration of a vehicle according to a gear shift line determined by an accelerator pedal operation amount and a vehicle speed using a continuously variable transmission.
Japanese Patent Application Publication No. 2000-2327 (JP 2000-2327 A) discloses a hybrid vehicle in which a stepped variable transmission is mounted. In JP 2000-2327 A, an input shaft of the stepped variable transmission is coupled to an output shaft of a power distribution mechanism (continuously variable transmission) so as to transmit torque. An output shaft of the stepped variable transmission is coupled to a drive shaft of the vehicle so as to transmit torque. That is, the continuously variable transmission and the stepped variable transmission are connected in series to each other.